Though Apple don't provide a corporate mobile email service, so there isn't the same kind of lock-in in this case, right?

No but Apple going out of business (what they fear of Blackberry, even if unlikely short term) would hinder their ability to deploy software, which is pretty bad too.

I would say Apple going out of business is unlikely long term! By the way: In-house developed apps are not depending on Apples App store.

http://www.apple.com/business/accelerator/

  I would say Apple going out of business is
  unlikely long term!

This is true. Apple has reached a Microsoft level, where it would take an act of god for the business to completely go under at this point. If it does go out of business, it will be gradually over the course of many years.

But they do depend on Apple's signing of the app before installation on the phone, right?

For development, Apple signs a certificate that lets you sign your own binaries, plus a provisioning profile that the phone uses to determine if it can run the app.

Presumably for the corporate version, they just give you a certificate you can use to sign, and one that you put on company phones so they're willing to run your software.

A lot of what's going on here is the time spent investigating what works, deciding what to support and documenting everything. That might not be literally "lock-in", but there's always a significant cost to move.

Given the amount of companies running Exchange, Apple is an inviting solution.

I would argue that these companies are pretty much locked into a Microsoft e-mail solution.

Yet most large enterprises are "locked in" to exchange, or worse lotus notes for email. If they've learned they hopefully learned to not depend on one phone vendor/platform and provide the ability for say, android, windows phone, bb, and ios devices to get to company resources.